An ion exchange membrane is used in many industrial fields, for example, as an electrodialytic membrane for use in a desalination step in salt production and food fields and an electrolyte film for fuel cells and as a membrane for diffusion dialysis used for acid collection from an acid containing a metal ion produced in the iron and steel industry. This ion exchange membrane has a structure that a base film serving as a reinforcing material is installed as a core material in an ion exchange resin, thereby providing certain film strength and film shape stability. If the core material is not existent, as the ion exchange resin has a large number of ion exchange groups, it readily swells when it is immersed in an electrolyte aqueous solution with the result of a reduction in strength and a form change.
It has been known that a porous thermoplastic resin film is used as the above base film, and this film is actually used. In an ion exchange membrane having this porous film as a base material, an ion exchange resin is filled in the pores of the porous film as the base material with the result that the electric resistance of the film (to be referred to as “membrane resistance” hereinafter) is advantageously low. For example, Patent Document 1 discloses a cation exchange membrane for salt production, comprising a porous stretched polyethylene film (Hipore of Asahi kasei E-Materials Corp. or SETELA of Tonen Chemical Nasu Corporation) as a base film.
By the way, since the ion exchange membrane is used for various purposes and different in implementation scale, it is various in size, and its strength, dimensional stability and shape stability must be improved according to the size.
Therefore, the thickness of the porous base film in the ion exchange membrane must be large. Since this porous film has a large number of pores, as compared with a film having no pores, it has low strength and no rigidity. Therefore, it must be made thick to improve its strength, dimensional stability and shape stability.
However, most commercially available porous films have a limited thickness of about several tens of μm. The production of porous films having a suitable thickness according to each purpose and size becomes small-quantity production, thereby losing an industrial advantage.
Then, commercially available porous films having a limited thickness are laminated, and this laminated film is used as a base film to produce an ion exchange membrane. In this case, means for laminating porous films becomes a problem.
For example, Patent Document 2 discloses anion exchange membrane which includes a porous film obtained by laminating two porous resin sheets and bonding them together by thermal fusion at a temperature higher than the melting point. Since the porous resin sheets are bonded together by thermal fusion at a temperature higher than the melting point, pores formed in the porous film are occluded. That is, this base film has low air permeability (high Gurley air permeance), and an ion exchange membrane formed by using this has high electric resistance. Therefore, in Patent Document 2, a fibrous resin sheet (specifically, a fibril-like sheet) is used as the porous resin sheet to be laminated so as to increase its porosity (void ratio), thereby suppressing a reduction in air permeability (a rise in Gurley air permeance) and a rise in the electric resistance of the ion exchange membrane derived from the occlusion of pores caused by thermal fusion.
Then, when a fibrous base film is used, porosity becomes too high with the result that the deterioration of mechanical strength cannot be avoided, thereby causing a problem that the thickness of the base film must be made larger than necessary. Therefore, an ordinary olefin resin base film cannot be used in this means and therefore, a polytetrafluoroethylene base film is used in Patent Document 2, resulting in extremely high cost.
In Patent Document 3, the present applicant proposes a process for producing an ion exchange membrane by filling a monomer composition for forming an ion exchange resin in spaces in a plurality of porous resin sheets (laminated sheet) which are laminated but not bonded together and polymerizing the monomer composition in this state to produce an ion exchange resin.
In this process, since the porous resin sheets are bonded together not directly but by means of the ion exchange resin filled in spaces (pores), a problem such as the occlusion of pores caused by the bonding of the porous resin sheets can be completely avoided, a rise in the electric resistance of the ion exchange membrane can be prevented without fail, and further, an expensive special resin such as polytetrafluoroethylene does not need to be used as the material of the porous resin sheet, resulting in a great economical advantage. However, in this process, since the porous resin sheets are not bonded together, interface delamination between the porous resin sheets readily occurs. Therefore, further improvement is needed.